CS520 Web Programming Spring – Inversion of Control Chengyu Sun California State University, Los Angeles

Background Originally developed by Rod Johnson Addresses many problems of EJB One of the most popular Java web development frameworks Books Expert One-on-One: J2EE Design and Development (2002) Expert One-on-One: J2EE Development without EJB (2004) Professional Java Development with the Spring Framework (2005)

Spring Framework

The Need for IoC The DAO Example The Data Access Object (DAO) pattern UserDao Example Model class Interface Implementation Usage in application code

Model Class public class User { private Integer id; private String username, password; private boolean enabled; }

DAO Interface public interface UserDao { User getUser( Integer id ); List<User> getUsers(); }

DAO Implementation Implement UserDao using JPA public class UserDaoImpl implements UserDao { private EntityManager entityManger; public User getUser( Integer id ) { return entityManager.find(User.class, id ); } ... ...

DAO Usage in Application Code UserController public class UserController { UserDao userDao; public String users( ModelMap models ) { List<User> users = userDao.getUsers(); … … }

Data Access Object (DAO) A Java EE design pattern Persistent Data Store DAO controller model view user Application code

Advantages of DAO Provide a data access API that is Independent of persistent storage types, e.g. relational DB, OODB, XML, flat files etc. Independent of persistent storage implementations, e.g. MySQL, PostgreSQL, Oracle etc. Independent of data access implementations, e.g. JDBC, Hibernate, etc.

Instantiate a UserDao Object in Application Code 1. UserDaoJpaImpl userDao = new UserDaoJpaImpl(); 2. UserDao userDao = new UserDaoJpaImpl(); Which one is better??

Problem Caused by Object Instantiation What if we decide to use JDBC instead of Hibernate/JPA, i.e. replace UserDaoJpaImpl with UserDaoJdbcImpl The application is not really independent of the data access method Switching to a different UserDao implementation affects all the code that instantiates UserDao

Another Way to Instantiate UserDao UserDao userDao; ... public void setUserDao( UserDao userDao) { this.userDao = userDao; } No more dependency on a specific implementation of the DAO But who will call the setter?

Inversion of Control (IoC) A framework like Spring is responsible for instantiating the objects and pass them to application code A.K.A. IoC container, bean container Inversion of Control (IoC) The application code is no longer responsible for instantiate objects like DAO, i.e. that “control” is taken way from the application code A.K.A. Dependency Injection

Example: Hello World Message is a Java object (or bean) managed by the Spring container Created by the container Property is set by the container

Bean Configuration File <beans> <bean id="msgBean“ class=“cs520.spring.hello.Message"> <property name="content" value="Hello World!" /> </bean> </beans> The string “Hello World” is injected to the bean msgBean

Understand Bean Container … Without a bean container Application Code Message object new Message() JVM

… Understand Bean Container With a bean container Application Code getBean(“msgBean”) <bean id="msgBean“ class="cs520.spring.hello.Message"> <property name=“content“ value="Hello World!" /> </bean> setContent(“Hello World!”); Bean Container Message object new Message() JVM

Dependency Injection Objects that can be injected Methods of injection Simple types: strings and numbers Collection types: list, set, and maps Other beans Methods of injection via Setters via Constructors

Dependency Injection Example DjBean Fields of simple types Fields of collection types Fields of class types

Quick Summary of Bean Configuration <bean>, “id”, “class” Simple type property <property>, “name”, “value” Class type property <property>, “name”, “ref” (to another <bean>) Collection type property <list>/<set>/<map>/<props>, <value>/<ref>/<entry>/<prop> Constructor arguments <constructor-arg>, “index”, same as other properties

Some Bean Configuration Examples <property name=“foo”> <map> <entry key=“key1”> <value>bar1</value> </entry> <entry key=“key2”> <ref bean=“bar2” /> </map> </property> <property name=“foo”> <set> <value>bar1</value> <ref bean=“bar2” /> </set> </property> <property name=“foo”> <props> <prop key=“key1”>bar1</prop> <prop key=“key2”>bar2</prop> </props> </property>

Wiring – The Stack Example (I) StackTest

Wiring – The Stack Example (II) StackTest ArrayList Stack2 LinkedList

Wiring – The Stack Example (III) StackTest ArrayList Stack2 LinkedList

Annotation-based Configuration Activate annotation processing with <context:annotation-config /> Automatically scan for Spring bean with <context:component-scan /> Mark a class to be a Spring bean with @Component Enable auto wiring with @Autowired

XML Namespace … <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd"> <context:annotation-config /> <context:component-scan base-package="cs520.spring.stack" /> </beans>

… XML Namespace <?xml version="1.0" encoding="UTF-8"?> <beans:beans xmlns=" http://www.springframework.org/schema/context " xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:beans="http://www.springframework.org/schema/beans" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd"> <annotation-config /> <component-scan base-package="cs520.spring.stack" /> </bean:beans>

<context:annotation-config> Activate the processing of a number of annotations, e.g. @Autowired @Qualifier @Resource @PersistenceContext

<context:component-scan> Scan all the classes under base-package and its sub-packages

Component Scanning @Component for regular bean classes @Repository for DAO classes @Controller for controller classes @Service for service classes

Auto Wiring Auto wire types For individual bean For all beans byName, byType, autodetect, constructor For individual bean <bean autowire=“autowire type”/> For all beans <beans default-autowrire=“autowire type”>

@Autowired The property does not need a setter Auto wired by type To auto wire by name Use @Qualifier Use @Resource

Advantages of IoC True separation of different components of an application Centralized bean dependency management Singleton objects improve performance Singleton vs. Prototype

Further Readings Spring in Action Chapter 1-3 Spring Framework Reference Documentation http://static.springsource.org/spring/docs/current/spring-framework-reference/html/ Chapter 5 The IoC Container